The investigator's objective is to develop and use non-invasive approaches to evaluate: (1) the effect of physical exercise on the growing and mature skeleton; and (2) to relate these changes to correlated changes in the skeletal muscles and their tendons. They will examine the extent to which phenotypic plasticity of the vertebrate musculoskeletal system in an integrated response to exercise. These animal experiments will help increase the understanding of the design principles of the musculoskeletal system common to all animals, including humans, and will have potential application in rehabilitation and training of children and adults associated with physical activity. The model the investigators will use involves quantification of the bone's loading history by recording in vivo strain at functionally equivalent sites on the bone at specific time during the exercise period. Correspondingly, the loading history of the muscles and their tendons are determined using force-platform/kinematic analyses and direct in vivo tendon force recordings. Exercise is varied by running the animals on a treadmill at different speeds and duration, and by varying the loads that the animals carry on their backs. Known parameters of the loading history for the bone, muscles and tendons can then be related to quantified changes in their morphology, focusing on bone formation and resorption rates at sites where the tissues's strain history is known. Specifically, the following three hypotheses will be tested: (1) that bone models to maintain a uniform distribution of strain at functionally equivalent sites; (2) that disruption of the normal distribution of strain, rather than elevated strain per se, is most critical for eliciting adaptive bone modeling; and (3) that the mass and form of bone, muscle and tendon in an animals's limb is matched to maintain a similar safety factor to failure, providing integrated design to protect against injury or failure. To test these hypotheses, the investigators will quantify bone, muscle and tendon loading during treadmill exercise in growing chicks and hamsters. Matched experiments will be carried out on mature animals to compare the adaptive plasticity of mature versus growing bone, muscle and tendon. They will also, evaluate more precisely the extent to which disruption of normal strain gradients in the bone's cortex affects adaptive modeling responses of the bone.